The invention relates to a method for burning lumpy combustion material, especially limestone, dolomite and magnesite, in a regenerative shaft furnace comprised of at least two shafts that comprise a preheating zone as well as a combustion zone, and which are connected above a cooling zone by a transfer channel, whereby
combustion air is delivered to the head of one shaft, heated by hot combustion material as it passes through the preheating zone, and brought into contact with fuel before it enters the combustion zone;
the combustion zone of the shaft is flowed through by combustion gas that is produced as the fuel is being burned;
combustion gas is passed through the transfer channel into the parallel shaft and flows through said parallel shaft from the bottom to the top, with transfer of heat to the combustion material present in the parallel shaft; and
cooling air is admitted into the lower end of the shaft and heated as it flows through the cooling zone by the hot combustion material exiting from the combustion zone.
In the process known from the German trade publication ZEMENT-KALK-GIPS [CEMENT-LIME-GYPSUM], No. 6, 1970, pages 277 to 284, which has found wide acceptance in practical life on account of its low energy requirements, the fuel is charged in the shafts via lances, which are suspended into the fill or introduced into hollow spaces located below refractory bridges arranged in the shaft at the end of the preheating zone. The preheating zone of one shaft is used as the regenerator. The combustion air is admitted at the top end of a shaft, flows through the preheating zone in a continuous current, is heated there in the hot fill of combustion material, and comes into contact with the fuel at the mouths of the lances, and combustion occurs. The combustion gas flows in a continuous current from the mouths of the lances to the transfer channel and is then received in the parallel shaft. From there, the combustion gas flows countercurrently in relation to the direction of movement of the combustion material into the preheating zone, in which it transfers excess heat to the combustion material charged in the parallel shaft. At fixed time intervals, after about 10 to 15 minutes in most cases, the feed of combustion air, the discharge of exhaust gas and the feed of fuel are reversed. The combustion air and the fuel are then fed into the parallel shaft and the exhaust gas is extracted at the top side of the first shaft.
In the known process, the cooling air, which is introduced into the underside of the cooling zones of both shafts, does not participate in the combustion process. The cooling air is heated as it flows through the cooling zone and admixed to the combustion gas introduced into the parallel shaft. The cooling air increases the amount of the exhaust gas current, which is extracted at the top side of the parallel shaft, and which, moreover, has a diluting effect. The effect of the admixed cooling air is that the CO2 content of the current of exhaust gas amounts to only 20 to 22% by volume.
Gas currents with a high CO2-content can be used as a valuable material, for example in the production of carbonate. Gas currents with a CO2-content of at least 30% by volume are required in the sugar and soda industries, as well as in the production of calcium precipitate. The exhaust gas collected in the burning of limestone, dolomite and the like in a regenerative shaft furnace is not suitable for such applications.
The invention is based on the problem of further developing the method specified above in such a way that the amount of air can be controlled in a flexible manner and particularly also controlled in a way such that the exhaust gas collected in the burning of limestone, dolomite and the like has a CO2-content in excess of 30% by volume, with no increase in the heat requirement of the regenerative shaft furnace.
Based on the method described above, the problem of the invention is solved according to the invention in that the combustion air heated in the preheating zone, and at least one partial current of the cooling air heated in the cooling air are extracted from the shaft and supplied together with fuel to a combustion chamber, in which the combustion gas for the combustion zone is produced. A very high CO2-content of the exhaust gas withdrawn from the regenerative shaft furnace can be achieved if the total cooling air is supplied to the combustion chamber and used as oxidant for the fuel.
Injectors operated with a preheated propellant gas are preferably used in order to suck the heated combustion air and the heated cooling air from the shaft and supply said heated combustion air and said heated cooling air to the combustion chamber. The propellant is usefully preheated to about 800xc2x0 to 1000xc2x0 C. in a central combustion chamber, or in decentralized combustion chambers.
The combustion chamber connected with the shaft for producing the combustion gas required in the combustion zone is in each case equipped with a burner or with a small number of burners, preferably two burners, which simplifies the feed of the fuel. The temperature in the combustion chamber is controlled by the flow of gas sucked off at the top end of the cooling zone. Using the cooling air for the combustion has the advantage that the air ratio of the regenerative shaft furnace can be adjusted and varied. Air ratio denotes in the present case the total amount of air supplied to the regenerative shaft furnace as cooling air and combustion air, based on the minimum amount of air required for a stoichiometric combustion. With a substantially preset volume of the stream of cooling air, the amount of combustion air introduced into the preheating zone can be adjusted in such a way that the air ratio xcex is smaller than 1.3. With an air ratio xcex=1.2, a CO2-content in the exhaust gas of about 34% by volume is adjusted in the exhaust gas of the regenerative shaft furnace operated according to the invention with natural gas firing and charging of a limestone with a CaCO3-content of about 98% by weight, and a residual CO2-content in the lime of about 1% by weight. The heat requirement of the furnace amounts to about 3,478 kJ/kg lime.
The combustion zone with flow in the same directions and counterflow can be fixed by the way in which the combustion chamber is arranged and the suction opening for the cooling air are positioned. In the combustion zone with continuous flow in the same direction as the combustion material, the combustion gas is guided in the direction of movement of the combustion material, and in the combustion zone with counterflow it is guided against the direction of movement of the combustion material. By changing the ratio of the zones with equi-flow and counterflow it is possible to influence the combustion process and to take into account the reactivity of the combustion material. Further variation possibilities ensue to that extent from the method as defined by the invention.
According to one implementation of the method as defined by the invention, provision is made that the combustion gas is introduced into the shaft above the transfer channel and flows through a combustion zone that reaches from the combustion chamber connected to the shaft, up to the transfer channel. The combustion zone of the shaft operates as a combustion zone with equi-flow, whereas the combustion zone of the parallel shaft is flowed through by the combustion gas countercurrently in relation to the direction of movement of the combustion material.
According to another implementation of the method as defined by the invention, the combustion gas is admitted at about the level of the transfer channel. The partial stream of combustion gas flows through a combustion zone of the shaft located below the transfer channel, with a direction of flow that is the same as the direction of movement of the combustion material, and is extracted from the shaft below the transfer channel with the cooling air heated in the cooling zone, and then recycled into the combustion chamber. The other part of the combustion gas is received in the parallel shaft by way of the transfer channel and flows through the combustion zone of said parallel shaft countercurrently in relation to the direction of movement of the combustion material. By controlling the process in the way described above, the combustion material is burned in a short equi-flow combustion zone located between the combustion chamber and the suction opening located below the transfer channel. However, the predominant part of the combustion process takes place in the combustion zone of the parallel shaft with counterflow.
Another possibility for varying the method as defined by the invention consists in that both shafts are equipped with combustion chambers, which are connected to the shafts at about the level of the transfer channel, whereby fuel is simultaneously admitted to said combustion chambers, together with a partial stream of the combustion air heated in the preheating zone of the first shaft. A part of the combustion gases introduced into the shafts from the combustion chambers flows through an equi-flow zone of the respective shaft located below the transfer channel, i.e. in the same direction as the direction of movement of the combustion material, is extracted from the shaft below the transfer channel with a stream of cooling air heated in the cooling-zone, and then recycled into the combustion chamber connected to the shaft. The other part of the combustion gas flows through a zone of counterflow of the second shaft, i.e. flows countercurrently in relation to the direction of movement of the combustion material. With such an implementation of the method as defined by the invention, the combustion material is simultaneously burned in both shaft in a continuous flow. Furthermore, a counterflow combustion zone is present in the shaft from which the exhaust gas is discharged at the top side, with the combustion material being burned in said counterflow combustion zone by combustion gas guided from the bottom upwards.
The described division in two equi-flow combustion zones and one counterflow combustion zone is achievable also if the combustion air heated in the preheating zone of the first shaft is admitted together with fuel into a combustion chamber that is connected to the transfer channel. A part of the combustion gas exiting from the combustion chamber flows in both shafts through a combustion zone located below the transfer channel, with flow in the same direction as the direction of movement of the combustion material, and is extracted from the shafts below the transfer channel with a stream of the cooling air heated in the cooling zone, and then recycled into the combustion chamber. The other part of the combustion gas flows through a counterflow combustion zone of the second shaft, i.e. it flows countercurrently in relation to the direction of movement of the combustion material.
Furthermore, the object of the invention is a regenerative shaft furnace according to claims 9 to 14 for carrying out the method described above.